In biology and ecology, extinction is the end of an organism or of a group of organisms (taxon), normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point. Because a species' potential range may be very large, determining this moment is difficult, and is usually done retrospectively. This difficulty leads to phenomena such as Lazarus taxa, where a species presumed extinct abruptly "re-appears" (typically in the fossil record) after a period of apparent absence.

Through evolution, new species arise through the process of speciation—where new varieties of organisms arise and thrive when they are able to find and exploit an ecological niche—and species become extinct when they are no longer able to survive in changing conditions or against superior competition. The relationship between animals and their ecological niches has been firmly established.[2] A typical species becomes extinct within 10 million years of its first appearance,[3] although some species, called living fossils, survive with virtually no morphological change for hundreds of millions of years. Most extinctions have occurred naturally, prior to Homo sapiens walking on Earth: it is estimated that 99.9% of all species that have ever existed are now extinct.[3][4]

Mass extinctions are relatively rare events; however, isolated extinctions are quite common. Only recently have extinctions been recorded and scientists have become alarmed at the high rates of recent extinctions.[5] Most species that become extinct are never scientifically documented. Some scientists estimate that up to half of presently existing species may become extinct by 2100.[6] It is difficult to estimate the trajectory that biodiversity might have taken without human impact but scientists at the University of Bristol estimate that biodiversity might increase exponentially without human influence.[2]

Definition

A species is extinct when the last existing member dies. Extinction therefore becomes a certainty when there are no surviving individuals that are able to reproduce and create a new generation. A species may become functionally extinct when only a handful of individuals survive, which cannot reproduce due to poor health, age, sparse distribution over a large range, a lack of individuals of both sexes (in sexually reproducing species), or other reasons.

In ecology, extinction is often used informally to refer to local extinction, in which a species ceases to exist in the chosen area of study, but still exists elsewhere. This phenomenon is also known as extirpation. Local extinctions may be followed by a replacement of the species taken from other locations; wolf reintroduction is an example of this. Species which are not extinct are termed extant. Those that are extant but threatened by extinction are referred to as threatened or endangered species.

An important aspect of extinction at the present time are human attempts to preserve critically endangered species, which is reflected by the creation of the conservation status"Extinct in the Wild" (EW). Species listed under this status by the International Union for Conservation of Nature (IUCN) are not known to have any living specimens in the wild, and are maintained only in zoos or other artificial environments. Some of these species are functionally extinct, as they are no longer part of their natural habitat and it is unlikely the species will ever be restored to the wild.[9] When possible, modern zoological institutions attempt to maintain a viable population for species preservation and possible future reintroduction to the wild through use of carefully planned breeding programs.

The extinction of one species' wild population can have knock-on effects, causing further extinctions. These are also called "chains of extinction".[10] This is especially common with extinction of keystone species.

Pseudoextinction

Descendants may or may not exist for extinct species. Daughter species that evolve from a parent species carry on most of the parent species' genetic information, and even though the parent species may become extinct, the daughter species lives on. In other cases, species have produced no new variants, or none that are able to survive the parent species' extinction. Extinction of a parent species where daughter species or subspecies are still alive is also called pseudoextinction.

Pseudoextinction is difficult to demonstrate unless one has a strong chain of evidence linking a living species to members of a pre-existing species. For example, it is sometimes claimed that the extinct Hyracotherium, which was an early horse that shares a common ancestor with the modern horse, is pseudoextinct, rather than extinct, because there are several extant species of Equus, including zebra and donkeys. However, as fossil species typically leave no genetic material behind, it is not possible to say whether Hyracotherium actually evolved into more modern horse species or simply evolved from a common ancestor with modern horses. Pseudoextinction is much easier to demonstrate for larger taxonomic groups.

Lazarus taxa

The coelacanth, a fish related to lungfish and tetrapods, was considered to have been extinct since the end of the Cretaceous Period until a specimen was found off the east coast of South Africa, off the Chalumna River (now Tyolomnqa) in 1938.[11] Museum curator Marjorie Courtenay-Latimer discovered the fish among the catch of a local angler, Captain Hendrick Goosen, on December 23, 1938.[11] A local chemistry professor, JLB Smith, confirmed the fish's importance with a famous cable: "MOST IMPORTANT PRESERVE SKELETON AND GILLS = FISH DESCRIBED".[11]

Far more recent possible or presumed extinctions of species which may turn out still to exist include the Thylacine, or Tasmanian Tiger (Thylacinus cynocephalus), the last known example of which died in Hobart Zoo in Tasmania in 1936; the Japanese Wolf (Canis lupus hodophilax), last sighted over a hundred years ago; the Ivory-Billed Woodpecker (Campephilus principalis), last sighted for certain in 1944; and the Slender-billed Curlew (Numenius tenuirostris), not seen since 2007.[12]

Causes

As long as species have been evolving, species have been going extinct. It is estimated that over 99.9% of all species that ever lived are extinct. The average life-span of most species is 10 million years, although this varies widely between taxa.
There are a variety of causes that can contribute directly or indirectly to the extinction of a species or group of species. "Just as each species is unique," write Beverly and Stephen C. Stearns, "so is each extinction ... the causes for each are varied—some subtle and complex, others obvious and simple".[13] Most simply, any species that cannot survive and reproduce in its environment and cannot move to a new environment where it can do so, dies out and becomes extinct. Extinction of a species may come suddenly when an otherwise healthy species is wiped out completely, as when toxicpollution renders its entire habitat unliveable; or may occur gradually over thousands or millions of years, such as when a species gradually loses out in competition for food to better adapted competitors. Extinction may take place a long time after the events that set it in motion, a phenomenon known as extinction debt.

Currently, environmental groups and some governments are concerned with the extinction of species caused by humanity, and are attempting to combat further extinctions through a variety of conservation programs.[5] Humans can cause extinction of a species through overharvesting, pollution, habitat destruction, introduction of invasive species (such as new predators and food competitors), overhunting, and other influences. Explosive, unsustainable human population growth is an essential cause of the extinction crisis.[14] According to the International Union for Conservation of Nature (IUCN), 784 extinctions have been recorded since the year 1500 (to the year 2004), the arbitrary date selected to define "modern" extinctions, with many more likely to have gone unnoticed (several species have also been listed as extinct since the 2004 date).[15]

Genetics and demographic phenomena

Population genetics and demographic phenomena affect the evolution, and therefore the risk of extinction, of species. Limited geographic range is the most important determinant of genus extinction at background rates but becomes increasingly irrelevant as mass extinction arises.[16]

Natural selection acts to propagate beneficial genetic traits and eliminate weaknesses. It is nevertheless possible for a deleterious mutation to be spread throughout a population through the effect of genetic drift.

Because traits are selected and not genes, the relationship between genetic diversity and extinction risk can be complex with factors such as balancing selection, cryptic genetic variation, phenotypic plasticity, and degeneracy all playing potential roles.

A diverse or deep gene pool gives a population a higher chance of surviving an adverse change in conditions. Effects that cause or reward a loss in genetic diversity can increase the chances of extinction of a species. Population bottlenecks can dramatically reduce genetic diversity by severely limiting the number of reproducing individuals and make inbreeding more frequent. The founder effect can cause rapid, individual-based speciation and is the most dramatic example of a population bottleneck.

Genetic pollution

Purebred wild species evolved to a specific ecology can be threatened with extinction[17] through the process of genetic pollution—i.e., uncontrolled hybridization, introgression genetic swamping which leads to homogenization or out-competition from the introduced (or hybrid) species.[18] Endemic populations can face such extinctions when new populations are imported or selectively bred by people, or when habitat modification brings previously isolated species into contact. Extinction is likeliest for rare species coming into contact with more abundant ones;[19]interbreeding can swamp the rarer gene pool and create hybrids, depleting the purebred gene pool (for example, the endangered Wild water buffalo is most threatened with extinction by genetic pollution from the abundant domestic water buffalo). Such extinctions are not always apparent from morphological (non-genetic) observations. Some degree of gene flow is a normal evolutionarily process, nevertheless, hybridization (with or without introgression) threatens rare species' existence.[20][21]

The gene pool of a species or a population is the variety of genetic information in its living members. A large gene pool (extensive genetic diversity) is associated with robust populations that can survive bouts of intense selection. Meanwhile, low genetic diversity (see inbreeding and population bottlenecks) reduces the range of adaptions possible.[22] Replacing native with alien genes narrows genetic diversity within the original population,[19][23] thereby increasing the chance of extinction.

Habitat degradation

Habitat degradation is currently the main anthropogenic cause of species extinctions. The main cause of habitat degradation worldwide is agriculture, with urban sprawl, logging, mining and some fishing practices close behind. The degradation of a species' habitat may alter the fitness landscape to such an extent that the species is no longer able to survive and becomes extinct. This may occur by direct effects, such as the environment becoming toxic, or indirectly, by limiting a species' ability to compete effectively for diminished resources or against new competitor species.

Habitat degradation through toxicity can kill off a species very rapidly, by killing all living members through contamination or sterilizing them. It can also occur over longer periods at lower toxicity levels by affecting life span, reproductive capacity, or competitiveness.

Habitat degradation can also take the form of a physical destruction of niche habitats. The widespread destruction of tropical rainforests and replacement with open pastureland is widely cited as an example of this;[6] elimination of the dense forest eliminated the infrastructure needed by many species to survive. For example, a fern that depends on dense shade for protection from direct sunlight can no longer survive without forest to shelter it. Another example is the destruction of ocean floors by bottom trawling.[24]

Diminished resources or introduction of new competitor species also often accompany habitat degradation. Global warming has allowed some species to expand their range, bringing unwelcome competition to other species that previously occupied that area. Sometimes these new competitors are predators and directly affect prey species, while at other times they may merely outcompete vulnerable species for limited resources. Vital resources including water and food can also be limited during habitat degradation, leading to extinction.

Predation, competition, and disease

In the natural course of events, species become extinct for a number of reasons, including but not limited to, extinction of a necessary host, prey or pollinator, inter-species competition, inability to deal with evolving diseases and changing environmental conditions (particularly sudden changes) which can act to introduce novel predators, or to remove prey. Recently in geologic time, humans have become an additional cause of extinction (many people would say premature extinction) for some species, either as a new mega-predator or by transportinganimals and plants from one part of the world to another. The later has been occurring for thousands of years, sometimes deliberately (e.g., livestock released by sailors onto islands as a source of future food) and sometimes accidentally (e.g., rats escaping from boats). In most cases, such introductions are unsuccessful, but when they do become established as an invasive alien species, the consequences can be catastrophic. Invasive alien species can affect native species directly by eating them, competing with them, and introducing pathogens or parasites that sicken or kill them or, indirectly, by destroying or degrading their habitat. Human populations may themselves act as invasive predators. According to the "overkill hypothesis", the swift extinction of the megafauna in areas such as Australia (40,000 years before present), North and South America (12,000 years before present), Madagascar, Hawaii (300-1000 CE), and New Zealand (1300-1500 CE), resulted from the sudden introduction of human beings to environments full of animals that had never seen them before, and were therefore completely unadapted to their predation techniques.[25]

Coextinction

Coextinction refers to the loss of a species due to the extinction of another; for example, the extinction of parasitic insects following the loss of their hosts. Coextinction can also occur when a species loses its pollinator, or to predators in a food chain who lose their prey. "Species coextinction is a manifestation of the interconnectedness of organisms in complex ecosystems ... While coextinction may not be the most important cause of species extinctions, it is certainly an insidious one".[26] Coextinction is especially common when a keystone species goes extinct.
Models suggest that coextinction is the most common form of biodiversity loss. Coextinction allows for cascading effects across the trophic levels. Such effects are most severe in mutualistic and parasitic relationships. An example of coextinction would be the Haast's Eagle and the Moa. The Haast's Eagle is an example of a predator that became extinct because its food source became extinct. The moa were several species of flightless birds that served as a food source to the Haast's Eagle.[27]

Climate change

Extinction as a result of climate change has been confirmed by fossil studies.[28] Particularly, the extinction of amphibians during the Carboniferous Rainforest Collapse, 350 million years ago.[28] A 2003 review across 14 biodiversity research centers predicted that, because of climate change, 15–37% of land species would be "committed to extinction" by 2050.[29][30][31] The ecologically rich areas that would potentially suffer the heaviest losses include the Cape Floristic Region, and the Caribbean Basin. These areas might see a doubling of present carbon dioxide levels and rising temperatures that could eliminate 56,000 plant and 3,700 animal species.[32]

There have been at least five mass extinctions in the history of life on earth, and four in the last 3.5 billion years in which many species have disappeared in a relatively short period of geological time. The massive eruptive event is considered to be one likely cause of the "Great Dying" about 250 million years ago,[33] which is estimated to have killed 90% of species existing at the time.[34] There is also evidence to suggest this event was preceded by another mass extinction known as Olson's Extinction.[33] The Cretaceous–Paleogene extinction event occurred 65 million years ago at the end of the Cretaceous period and is best known for having wiped out non-avian dinosaurs, among many other species.

Modern extinctions

According to a 1998 survey of 400 biologists conducted by New York's American Museum of Natural History, nearly 70 percent believed that they were currently in the early stages of a human-caused extinction,[35] known as the Holocene extinction. In that survey, the same proportion of respondents agreed with the prediction that up to 20 percent of all living populations could become extinct within 30 years (by 2028). Biologist E. O. Wilson estimated [6] in 2002 that if current rates of human destruction of the biosphere continue, one-half of all species of life on earth will be extinct in 100 years.[36] More significantly the rate of species extinctions at present is estimated at 100 to 1000 times "background" or average extinction rates in the evolutionary time scale of planet Earth.[37] However, some groups are going extinct much faster. Amphibians, for example are disappearing as much as 45,000 times their extinction rate at K-Pg. [38] It is surprising that with such extreme losses on a global scale that the public's interest in extinction is experiencing a world-wide decline. [39]

History of scientific understanding

When it was first described in the 1750s, the idea of extinction was threatening to those who held a belief in the Great Chain of Being, a theological position that did not allow for "missing links".[40]

The possibility of extinction was not widely accepted before the 1800s.[40][41] The devoted naturalist Carl Linnaeus could "hardly entertain" the idea that humans could cause the extinction of a species.[42] When parts of the world had not been thoroughly examined and charted, scientists could not rule out that animals found only in the fossil record were not simply "hiding" in unexplored regions of the Earth.[43]Georges Cuvier is credited with establishing extinction as a fact in a 1796 lecture to the French Institute.[41] Cuvier's observations of fossil bones convinced him that they did not originate in extant animals. This discovery was critical for the spread of uniformitarianism,[44] and led to the first book publicizing the idea of evolution [45] though Cuvier himself strongly opposed the theories of evolution advanced by Lamarck and others.

Human attitudes and interests

Extinction is an important research topic in the field of zoology, and biology in general, and has also become an area of concern outside the scientific community. A number of organizations, such as the Worldwide Fund for Nature, have been created with the goal of preserving species from extinction. Governments have attempted, through enacting laws, to avoid habitat destruction, agricultural over-harvesting, and pollution. While many human-caused extinctions have been accidental, humans have also engaged in the deliberate destruction of some species, such as dangerous viruses, and the total destruction of other problematic species has been suggested. Other species were deliberately driven to extinction, or nearly so, due to poaching or because they were "undesirable", or to push for other human agendas. One example was the near extinction of the American bison, which was nearly wiped out by mass hunts sanctioned by the United States government, to force the removal of Native Americans, many of whom relied on the bison for food.[46]

Biologist Bruce Walsh of the University of Arizona states three reasons for scientific interest in the preservation of species; genetic resources, ecosystem stability, and ethics;[47] and today the scientific community "stress[es] the importance" of maintaining biodiversity.[47][48]

In modern times, commercial and industrial interests often have to contend with the effects of production on plant and animal life. However, some technologies with minimal, or no, proven harmful effects on Homo sapiens can be devastating to wildlife (for example, DDT).[49]BiogeographerJared Diamond notes that while big business may label environmental concerns as "exaggerated", and often cause "devastating damage", some corporations find it in their interest to adopt good conservation practices, and even engage in preservation efforts that surpass those taken by national parks.[50]

Governments sometimes see the loss of native species as a loss to ecotourism,[51] and can enact laws with severe punishment against the trade in native species in an effort to prevent extinction in the wild. Nature preserves are created by governments as a means to provide continuing habitats to species crowded by human expansion. The 1992 Convention on Biological Diversity has resulted in international Biodiversity Action Plan programmes, which attempt to provide comprehensive guidelines for government biodiversity conservation. Advocacy groups, such as The Wildlands Project[52] and the Alliance for Zero Extinctions,[53] work to educate the public and pressure governments into action.

People who live close to nature can be dependent on the survival of all the species in their environment, leaving them highly exposed to extinction risks. However, people prioritize day-to-day survival over species conservation; with human overpopulation in tropical developing countries, there has been enormous pressure on forests due to subsistence agriculture, including slash-and-burn agricultural techniques that can reduce endangered species's habitats.[54]

The American philosopher Michael Levin argues, "The very fact that a species is near extinction implies that its final demise will have negligible impact,"[55] though this discounts the damage done to reach that point.

Planned extinction

Implemented

The smallpox virus is now extinct in the wild,[56] although samples are retained in laboratory settings.

The polio virus is now confined to small parts of the world due to extermination efforts.[57]

Proposed

Olivia Judson has advocated the deliberate extinction of specific species. Her September 25, 2003 New York Times article, "A Bug's Death", advocates "specicide" of thirty mosquito species through the introduction of a genetic element, capable of inserting itself into another crucial gene, to create recessive "knockout genes". Her arguments for doing so are that the Anopheles mosquitoes (which spread malaria) and Aedes mosquitoes (which spread dengue fever, yellow fever, elephantiasis, and other diseases) represent only 30 species; eradicating these would save at least one million human lives per annum at a cost of reducing the genetic diversity of the familyCulicidae by only 1%. She further argues that since species become extinct "all the time" the disappearance of a few more will not destroy the ecosystem: "We're not left with a wasteland every time a species vanishes. Removing one species sometimes causes shifts in the populations of other species — but different need not mean worse." In addition, anti-malarial and mosquito control programs offer little realistic hope to the 300 million people in developing nations who will be infected with acute illnesses this year. Although trials are ongoing, she writes that if they fail: "We should consider the ultimate swatting."[58]

Cloning

Ongoing technological advances have encouraged the hypothesis that by using DNA from the remains of an extinct species, through the process of cloning, the species may be "brought back to life". Proposed targets for cloning include the mammoth, thylacine, and the Pyrenean Ibex. For such a program to succeed, a sufficient number of individuals would have to be cloned, from the DNA of different individuals (in the case of sexually reproducing organisms) to create a viable population. Though bioethical and philosophical objections have been raised,[59] the cloning of extinct creatures seems a viable outcome of the continuing advancements in science and technology.[60]

In 2003, scientists attempted to clone the extinct Pyrenean Ibex (C. p. pyrenaica). This initial attempt failed; of the 285 embryos reconstructed, 54 were transferred to 12 mountain goats and mountain goat-domestic goat hybrids, but only two survived the initial two months of gestation before they too died.[61] In 2009, a second attempt was made to clone the Pyrenean Ibex; one clone was born alive, but died seven minutes later, due to physical defects in the lungs.[62]

The concept of cloning extinct species was thought to be first popularized by the successful 1990 Michael Crichton novel and subsequent film Jurassic Park, though it may have been first used in John Brosnan's 1984 novel Carnosaur, then in F. Paul Wilson's 1989 novel Dydeetown World, and later in Piers Anthony's 1990 novel Balook, which featured the resurrection of a Baluchitherium, though Pat Mills' Judge Dredd story "The Cursed Earth" – in which the titular lawman battles tyrannosaurs who live wild in post-apocalyptic America after they escape from the theme park where they have been cloned to be used as attractions - precedes these examples, published as it was in 1978.

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